1. Field of the Invention
The present invention relates to a clamp of the type adapted to fix, for example a tool holder to a spindle of a machining center by means of a spring force, and more specifically to a device for holding a fixed member (such as the tool holder and the like) under a clamped condition even in the case that an unclamping force larger than the spring force is imposed to the fixed member.
2. Description of Prior Art
As such a spring clamp known by the inventors of the present invention, there has been provided the one disclosed in the Japanese Utility Model Laid Open Publication No. 1987 -150035.
This clamp has the following basic construction as illustrated in FIG. 18.
As illustrated in FIG. 18(a), a clamping actuation member B is supported by means of a cylindrical housing A in a vertically movable manner. The clamping actuation member B has been actuated for clamping to an upper clamping position X by means of a clamping spring 103. To the contrary, as illustrated in FIG. 18(c), the clamping actuation member B has been lowered to a lower unclamping position Z by means of a pressing-down force exerted by an unclamping actuation means D against the clamping spring 103. Along the extent of the housing A and the actuation member B there are provided a clamped condition holding device 108 and a changeover device 109. The clamped condition holding device 108 is adapted to be changed over by means of the changeover device 109 between the locked condition R and the unlocked condition U. Under the locked condition R illustrated in FIG. 18(a), the actuation member B is held in the clamping position X, and to the contrary, its clamped condition is cancelled under the unlocked condition U illustrated in FIGS. 18(b) and 18(c).
In the above-mentioned basic construction, conventionally the clamped condition holding device 108 and the changeover device 109 are constructed as follows.
As illustrated in FIG. 18, the clamped condition holding device 108 includes steel balls 112 inserted into a plurality of guide ports 111 formed in the upper portion of the cylindrical wall of the housing A. The actuation member B is provided at its upper portion with a ball receiving groove 113, a ball butting portion 114 and a ball pushing out step-up portion 115 in order from below.
Then, the changeover device 109 comprises a changeover cylinder 117 resiliently urged upward with respect to the housing A by means of a return spring 116. The changeover cylinder 117 is provided at its inner surrounding surface with a ball pushing out tapered surface 118 and a ball retreating groove 119 in order from below.
The spring clamp having such conventional construction operates as follows.
Under the clamping condition illustrated in FIG. 18(a). the balls 112 are pushed inwardly by means of the pushing out tapered surface 118 of the changeover cylinder 117 and then butted against the butting portion 114, so that the clamped condition holding device 108 has been changed over to the locked condition R. Thereby, the actuation member B can be held in the clamping position X even though a pulling down force larger than a resilient force of the clamping spring 103 is imposed thereto.
At the time of unclamping, the changeover cylinder 117 is lowered by means of the unclamping actuation means D. Thereupon, firstly as illustrated in FIG. 18(b), as the changeover cylinder 117 is lowered by a locked condition cancellation stroke T, the balls 112 becomes so repositioned as to face the retreating groove 119 relative to the changeover cylinder 117. Thereby, as illustrated in FIG. 18(c), the balls 112 are pushed outwardly by means of the pushing out step-up portion 115, so that the clamped condition holding device 108 has become possible to be changed over to the unlocked condition U. Under this condition, the actuation means D acts to lower the actuation member B by an unclamping stroke S against the clamping spring 103 so as to change over the actuation member B to the unclamping position Z.
There are, however, the following problems associated with the above-mentioned prior art.
Since the balls 112 supports the butting portion 114 through a point contact respectively, an allowable butting force can't help becoming small. Therefore, a clamping capability of the clamped condition holding device 108 is small correspondingly.
Further, since the balls 112 are apt to be plastically deformed by a large surface pressure produced by the point contact, their smooth advancing and retreating movements within the guide ports 111 become obstructed soon. Therefore, the service life of the clamped condition holding device 108 is short.
Furthermore, since the locked condition cancellation stroke T is required to have a large distance substantially corresponding to the diameter of the ball 112, the stroke of the unclamping actuation means D becomes long. As a result, the operation time at the time of unclamping is long.